The objective of this project is to describe, assess, and interpret mechanisms of breeding behavior and decision-making in a natural animal population. Humans and other animals make decisions concerning where to engage in different activities and these decisions affect subsequent choices of mates, dwelling and reproductive areas, escape routes, and foraging areas. Since the "mental health", well-being, and stress levels of an organism may reflect the effectiveness of these decision-making processes, there is strong reason to believe that enlarging our pool of knowledge concerning mechanisms of decision-making about crucial life factors, such as habitat, mates, and territories will eventually contribute to clinical knowledge and treatment. Specifically, the study is based on monitoring site selection, territoriality, hormonal function, and reproductive success in a large, color-marked population of Red-winged Blackbirds. This long-term monitoring of known individuals, combined with manipulative experiments that would be impossible with humans, will provide data on how males obtain breeding territories and what information they use to maximize their chances of becoming territorial. In addition, field monitoring of plasma steroid hormone levels, especially those of testosterone and corticosterone, will reveal how hormonal states influence behavior during the breeding season and how behavior, in turn, influences circulating hormone levels. Lastly, DNA fingerprinting analysis will reveal the frequency of extra-pair fertilizations and the proportion of offspring sired by other than putative fathers, and allow us to explore the question of how strongly parent-offspring genetic relationships are correlated with mating behavior.